customer decides the price

this is my first year on my own in the past i worked for other lawn crews. i have this one house and for some reason the woman decided that after it was cut that the hole thing did not need to be cut.. so she lowered the price on the bill..... with out contacting me.. this is bull sh*t what should i do she already sent me a check and all

Call her, very politely tell her your position, and that she needs to look for someone else to do her lawn cause you won't be back, and no you don't know anyone to recommend, and no matter what she says/offers, DON"T GO BACK.

You can take the advice above and waste all sorts of time, energy, and mental energy on trying to get your extra $20 back.

OR

You can just do what Potchins said (Fire her), learn from it, and then don't even give her another thought. Pick yourself up. Dust yourself off. And start all over again - with a new one.

Seriously, it's not worth wasting any time or energy at all on people like this. Get on with life and focus on the good clients who get it. Because there are hundreds of them out there just looking for you.

I have run in to individuals like this before.
I would suggest following Jim Lewis's advice.
Additionally, the worst possible thing you can do is take it personally.
For the $20.00, I wouldn't pursue her - despite it being a matter of principle. The courts won't see it that way regardless.